The invention relates to a resistive short-circuit current limiter having a support body which is at least partially composed of electrically insulating material, having at least one conductor track structure with high-Tc superconductor material, which structure is arranged on the support body and contains at least one conductor track having a plurality of first conductor track segments extending essentially rectilinearly and having a plurality of second conductor track segments extending non-rectilinearly, and also having end pieces of the conductor track structure for the latter to make contact with connecting conductors. The invention furthermore relates to a method for producing such a short-circuit current limiter. A corresponding short-circuit current limiter and also a method for producing it are described in European published patent application EP 0 523 374 A.
In electrical alternating-current supply systems, it is not possible reliably to avoid short circuits and electrical flashovers. The alternating current in the affected circuit then rises very quickly, i.e. in the first half-cycle of the current, to a multiple of its nominal value until it is interrupted by suitable protection and/or switching means. As a consequence of this, considerable thermal and mechanical stresses due to electromechanical forces occur in all system components affected, such as lines and busbars, switches or transformers. Since these short-term loads increase with the square of the current, reliable limiting of the short-circuit current to a lower peak value can considerably reduce the requirements made of the load-carrying capability of these system components. As a result, cost advantages can be achieved, for instance when constructing new systems and extending existing systems, in that a replacement of system components by embodiments with higher load-carrying capability can be avoided by installing current limiters.
Using superconducting short-circuit current limiters of the resistive type, the current rise after a short circuit can be limited to a value of a few multiples of the nominal current; furthermore, such a limiter is operational again a short time after disconnection. Thus, it acts like a fast self-healing fuse. It also ensures high operational reliability since it acts passively, i.e. operates autonomously without previous detection of the short circuit and without active triggering by a switching signal.
Resistive superconducting short-circuit current limiters of the type mentioned in the introduction form a superconducting break to be inserted serially into a circuit. In this case, the transition of a superconducting conductor track structure of this limiter from the practically resistanceless cold operating state below the critical temperature Tc of the superconductor material used into the normally conductive state above Tc (so-called phase transition) is utilized, in which case the electrical resistance Rn then present in the conductor track structure limits the current to an acceptable magnitude I=U/Rn. The heating above the critical temperature Tc is done by Joule heat in the superconductor material of the conductor track structure itself when, after a short circuit, the current density j rises above the critical value jc of the superconductor material, where the material may already have a finite electrical resistance even below the critical temperature Tc. In the limiting state above the critical temperature Tc, a residual current continues to flow in the circuit containing the short-circuit current limiter, until an additional mechanical isolator completely interrupts the circuit.
Superconducting short-circuit current limiters with known metal-oxide high-Tc superconductor materials (abbreviated to HTS materials), the critical temperature Tc of which is so high that they have to be kept in the superconducting operating state using liquid nitrogen of 77K, exhibit a fast increase in the electrical resistance when the critical current density jc is exceeded. The heating during the transition into the normally conductive state and thus the current limiting occur in a comparatively short time, so that the peak value of the short-circuit current can be limited to a fraction of the unlimited current, for instance to 3 to 10 times the nominal current. The superconducting current path of the short-circuit current limiter is in contact with a coolant which is capable of returning it to the superconducting operating state in a comparatively short time after the critical current density jc has been exceeded.
The resistive short-circuit current limiter which operates correspondingly and can be found in the above-mentioned publication EP 0 523 374 A contains a conductor track structure in the form of an electrical conductor made of an HTS material. The conductor is wound in a meandering shape and, for example, is worked from a 5 mm thick plate of the HTS material by means of lateral slotting. The conductor is therefore composed of first conductor track segments, which extend practically rectilinearly and are parallel to one another, and conductor track segments which extend transversely with respect to the rectilinear conductor track segments and connect the latter. The conductor thus forms a bent or angled conductor track in the mutual connection region at the end of the first, rectilinear conductor track segments, which conductor track can be regarded there as a second conductor track segment extending non-rectilinearly. The entire conductor may be designed to be self-supporting; however, it may also be situated on a support body in order to increase its mechanical stability. The end pieces of the conductor are designed as contact pieces to which external connecting conductors can be connected for the purpose of feeding in the current to be limited.
In the case of a superconducting short-circuit current limiter of this type, the problem arises that at the bend points of the conductor track between a rectilinearly extending (first) conductor track segment and the conductor track segment extending transversely with respect thereto and connecting such rectilinear conductor track segments, the current distribution and thus the thermal loading in the limiting case are inhomogeneous. It has therefore been considered to be necessary heretofore to use the conductor track of known short-circuit current limiters only for correspondingly lower switching capacities; or it was necessary to overdimension the non-rectilinear (second) conductor track segments such that they did not have an undesirable power limiting function. In the latter case, however, the area utilization of the surface of the support body was then correspondingly unfavorable.
The object of the present invention is to provide a short-circuit current limiter with a conductor track formed of high-temperature superconductor material of the type mentioned in the introduction which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and to the effect that it can be used to provide a comparatively higher maximum switching capacity, without necessitating over-dimensioning of conductor track segments. It is a further object of the invention to specify a method for the simplest possible production of such a limiter.
With the above and other objects in view there is provided, in accordance with the invention, a resistive short-circuit current limiter, comprising:
a support body at least partially composed of electrically insulating material;
a conductor track structure with high-Tc superconductor material disposed on the support body, the conductor track structure having at least one conductor track with a plurality of first conductor track segments extending substantially rectilinearly, a plurality of second conductor track segments extending non-rectilinearly, and end pieces of the conductor track structure for contacting connecting conductors;
the second conductor track segments forming an arc with a maximum radii ratio v=ra/ri of 7, where ra is an outer radius and ri an inner radius of the arc of the respective conductor track segment.
There is also provided, in accordance with the invention, a method of producing such a resistive short-circuit current limiter. In the method, a layer of high-Tc superconductor material is deposited directly or indirectly on the support body, and the layer is patterned to form a conductor track structure having at least one conductor track with a plurality of first conductor track segments extending substantially rectilinearly and a plurality of second conductor track segments extending non-rectilinearly along an arc having an outer radius ra and an inner radius ri and a maximum radii ratio v=ra/ri of 7.
According to the invention, the above objects are achieved with respect to the short-circuit current limiter in that all the second conductor track segments extending non-rectilinearly are configured arcuately with a maximum radii ratio v=ra/ri of 7. Transition pieces or end pieces at the ends of the at least one conductor track for making contact with connecting conductors are in this case not intended to be included in the dimensioning, according to the invention, of the maximum radii ratio.
The particular advantages associated with this configuration of the short-circuit current limiter are to be seen in the fact that upon maintaining the specified geometrical criterion for the second, arcuate conductor track segments, the inhomogeneous local loading is reduced to a tolerable amount, without significantly impairing the area utilization. This is because the excessive increase in temperature at the internal radius relative to the external radius T(ri)/T(ra) during the switching of the corresponding conductor track segment is limited to a maximum of v2.
Preferably, a maximum radii ratio v=ra/ri of 6, in particular of 5, is chosen. At these values, the risk of an undesirable temperature increase can practically be precluded for customary conductor track widths.
For reasons of good area utilization, it is particularly favorable if all the second, arcuate conductor track segments have a (minimum) radii ratio v of at least 2.5, preferably of at least 3. Although smaller values of v, and hence larger internal radii, reduce the excessive increase in temperature, they also impair the area utilization.
For the same reasons, all the second, arcuate conductor track segments advantageously have at least approximately the same conductor track width as the first, rectilinear conductor track segments.
Consequently, in practice, a range of the radii ratio v of all the second conductor track segments of between 3 and 6 represents a good compromise between excessive increase in temperature and area utilization.
A support body made of a special glass material is advantageously chosen for the short-circuit current limiter according to the invention, particularly when a large-area construction is involved.
An advantageous method for producing a short-circuit current limiter according to the invention is wherein the at least one conductor track is formed by patterning at least one layer which is made of the high-Tc superconductor material and is applied directly or indirectly on the support body. The individual conductor track segments can be designed in a comparatively simple manner using conventional physical, mechanical and/or chemical patterning processes.
Further advantageous configurations of the short-circuit current limiter according to the invention and, respectively, of the method for producing it emerge from the dependent claims in each case.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a resistive short-circuit current limiter having a conductor track structure made of high-Tc superconductor material, and method for producing the current limiter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.